What about Americans impresses first-time visitors the most?

I should be preface this by saying these were the perceptions of a child (as I was when I lived there), albeit one who had lived in other major cities by the time I arrived in America...

The size. Of everything. So my first landing in the U.S. was New York (Manhattan). I'd never seen skyscrapers so large, the canyons of streets, the size of the crowds. Even the size of some individual people - much larger ( I mean height, not girth) in some cases than I was used to.
For another example, the local toy shop (Toys 'R' Us) was vaster by order of magnitudes than any toy shop I'd ever been in.

One aside, Central Park did not actually 'wow' me as I was coming from Dublin where we have one of the largest urban parks in Europe (the Phoenix Park http://www.phoenixpark.ie/about/) but that was a rare anomaly.

A full pizza was about 10 times the size of the miserable little discs just becoming available in Ireland at the time...my father brought me to a restaurant famous for its 2 pound hamburgers (I wish I could remember the name, but the 2 pounder was its USP if any one recalls such a place in Manhattan in the late 70's/early 80's? I'm guessing lower East Side...). It was more meat on a plate at one time than I had ever seen....
The doorman of our apartment building was probably the largest (again in terms of height and breadth) human I had ever seen in real life up until that point...

I could go on and on but hopefully you get the picture...

Later we went on holiday to California where again I was shocked into silence just at the size of this strange, wonderful country, the time it took to get there and the huge vistas once we were there.

[Edit]: Wups! A little off topic, I dunno if the question has been edited or I just messed up and misread it but I answered for 'America' instead of 'Americans'.
I don't know my answer is not still relevant in part but if I had to answer on 'Americans' instead of the place that shapes them, I'd say:

  • Their optimism. In America and outside of the country I have always been awed and a little humbled at what I percive to be the 'American spirit' - indefatigable, can do, nothing is impossible, etc.

  • Their innocence, partially related to the above. Please note: I do not mean this in a cynical, pejorative or denigrating manner.

In my experience the 'default setting' of most Americans is to accept things in the manner in which they are presented and to trust what they are told.
This is A Good Thing.
Jaded Euro-types can snigger into their absinthe and Gauloises all they like but this is - for me - the single most endearing and impressive quality of Americans and I would despair should they have this abraded or degraded through contact with sarcastic Old World types with a false and wholly undeserved belief that their sneering 'sophistication' is better than an open, trusting and straightforward nature. 

I am not saying there aren't black-polo-necked sneering cynics with American passports, or 'can do' types with Euro or Asian ones but the above is my takeaway from having lived there, been in a long relationship with an American woman and in general experience.
How polite and friendly people are.

(I'm from the UK.)

The first thing that impressed me: the first time I came to the US was in 1996, the flight was delayed, and we were put in a hotel.  It was the middle of the night, there was a van driver who drove us to the hotel.  He was very polite.  I got on, he asked how I was doing.  My thoughts went something like "What?  How am I doing?  It's the middle of the night, I'm on a late flight, I'm rather exhausted, and I don't know where I am.  You're driving a van, in the middle of the night, you're probably not earning a great deal driving pissed off travelers back and forth between a hotel.  And you care how I'm doing??!?" 

I didn't actually say that.  I was so shocked  to be asked that I was speechless.  And so tired that I couldn't have coalesced my thoughts into a coherent sentence.

Now I live in the US.  When I go back to the UK, I wonder what's wrong with all the people working in stores that they're so rude, miserable and pissed off. Then I realise it's OK, they're British.
The fact that nobody walks anywhere and is surprised when tourists try. We asked two nice old ladies at the Smithsonian for directions somewhere and they pulled out a map and spent ages telling us how to get to where we wanted to go. They said it wouldn't take too long but parking would be a problem. We said that wouldn't be an issue because we were walking and they were stunned and said it was too far to walk. They called over another lady who backed them up and the three of them tried to talk us out of our ludicrous notion and almost begged us to take a taxi. We walked. It took about 50 minutes.
If Britain and France had undertaken a major offensive against Germany in September, 1939, how would it have turned out?

No. The British didn't even have the BEF in position until it was too late. The French would have had to get their act together and mount a violent and well supported attack against the Saar. Now that just wasn't in the cards, as